enable alpha channel support in UI

[jvonitter]

Investigate and implement transparency control in the browser UI.

Additional background from Kip: The front-end JS code and CSS in Firefox may also need to be extended, effectively creating a “vr optimized theme” for Firefox using the existing mechanisms used for the 2d window. A simple chroma-key or added alpha channel could be used to specify a non-square outline. If/when the JS customizations become substantial, we could consider distributing them within the steam deployed bundle and pass the appropriate flags to Firefox.exe to override the default front-end with the externally provided content.

[jvonitter]

fixes #26 - if we do this, we dont need to update the overlay UI